The objectives of this research proposal is to investigate the physiological sites and mechanisms of plasma protein catabolism. Our first goal is to identify the tissue and cell types where catabolism occurs. We have developed a novel radioactive label for proteins which permits the determination of the sites of clearance of proteins with the long circulating half-lives characteristic of plasma proteins. Our preliminary experiments indicate that covalent attachment of the trisaccharide, 3H-raffinose (RAF) to proteins alters neither their half-lives in the circulation nor their organ and cellular sites of catabolism; in addition, the 3H-RAF accumulates in, and identifies, the cell in which the labeled protein is degraded. We will use the 3H-RAF label to determine, in the rat, the sites of catabolism of rat albumin and immunoglobulins IgG and IgM, which are representative of the major classes of plasma proteins, and which are each cleared by apparently different mechanisms at, as yet, unknown sites. Following injection of 3H-RAF-labeled proteins we will monitor, with time, the tissue sites of accumulation of the RAF label and thus the sites of protein degradation. Once we have identified the in vivo sites of protein catabolism we will initiate studies concerning the mechanisms regulating plasma protein clearance from the circulation. In addition to naturally occuring plasma proteins, we will also study the sites of degradation of 3H-RAF-labeled bovine pancreatic isozymes RNase A and RNase C. These isozymes, which differ only by the presence of a complex, acidic oligosaccharide on RNase C, also have long circulating half-lives when injected into nephrectomized rats. Study of these two proteins will allow us to assess, for the first time, the role of the attached, complex carbohydrate in determining the site of catabolism of plasma type glycoproteins.